The present invention concerns a method for measuring the wear of a tire fitted on an automobile vehicle, while the vehicle is rolling or at rest, a suspension system designed for fitting on such a vehicle to perform the method, and a vehicle comprising a suspension system in accordance with the invention.
It is known to provide detectors of the wear of the tread patterns of tires fitted on an automobile vehicle to measure the wear of the tires during rolling.
German patent DE-A-197 45 734 (see FIGS. 2 and 3 thereof) discloses a tire whose tread comprises in its mass a plurality of metallic wires which form electrically conducting loops that extend respectively to different heights within a pattern rib of the tread, and which are connected to a detection circuit underneath the rib. As the tread of this tire is worn away, as will occur as a vehicle fitted with this tire is driven, these loops are broken one after the other to form open switches and the detection circuit delivers a signal representative of these broken loops to an evaluation unit present in the vehicle.
A major disadvantage of this wear detector is the complexity involved in the fabrication of the tire incorporating such a device in its mass, and the possible changes in the detector's components caused by the mechanical stresses and heating that are inevitable as the tire rolls.
One purpose of the present invention is to propose a method for measuring the wear of a tire having a tread with pattern elements interconnected by grooves and mounted on a wheel fitted to an automobile vehicle comprising a chassis, and a suspension system which forms a flexible link between the chassis and the wheel, the method overcoming the above disadvantages and allowing the wear to be measured at any time, whether continuously while the vehicle is rolling or when it is at rest.
To this end, the wear measurement method according to the invention, with the vehicle rolling or at rest, includes the following:    i) at a given moment a value is measured, which is representative of the distance between, on the one hand, a first point fixed on the suspension system facing the tire and occupying a position or a plurality of positions such that the orthogonal projection of the first point on the median circumferential plane of the tire remains an equal distance from that of the axis of the wheel on the plane, or a second point fixed on the chassis facing the tire, and, on the other hand, a point at the top of at least one of the tread pattern elements opposite the first or second point,    ii) in a first case, at the given moment a reference value is measured which is representative of a distance measured between the first or second point and the bottom of one of the grooves opposite the first or second point, or, alternatively, in a second case, a subsequent measurement is made of another reference value representative of a distance measured between the first point and the point at the top of the at least one tread pattern element, and    iii) the value measured in paragraph i) is compared with the reference value measured in paragraph ii) and from this is deduced, in the first case a wear datum representing the height of the tread pattern element by differential measurement at the moment, and in the second case a wear datum representing the change of wear at the top of the element within a given time interval.
According to a first embodiment of a method of the invention, the method comprises the following steps:    i) at a given moment a value is measured, which is representative of the distance between a first point fixed on the suspension system and a point at the top of at least one of the tread pattern elements opposite the first point,    ii) in a first case, at the given moment a reference value is measured, which is representative of a distance measured between the first point and the bottom of one of the grooves opposite the first point, or, alternatively, in a second case, a subsequent measurement is made of another reference value representative of a distance measured between the first point and one of the points at the top of at least one of the tread pattern elements, and    iii) the value measured in paragraph i) is compared with the reference value measured in paragraph ii), and from this is deduced, in the first case a wear datum representing the height of the pattern element by differential measurement at the moment, and in the second case a wear datum representing the change in wear at the top of the element within a given time interval.
According to a second embodiment of the invention, the method comprises the following steps:    i) at a given moment a value is measured, which is representative of the distance between the second point fixed on the chassis and a point at the top of at least one of the tread pattern elements opposite the second point,    ii) at the same given moment a reference value is measured, which is representative of a distance measured between the second point and the bottom of one of the grooves opposite this second point, and    iii) the value measured in paragraph i) is compared with the reference value measured in paragraph ii), and from this is deduced a wear datum representing the height of the pattern element by differential measurement at the moment.
According to another embodiment of the invention, the method comprises the following steps:    i) at a given moment a value is measured, which is representative of the distance between, on the one hand the first point fixed on the suspension system or the second point fixed on the chassis and, on the other hand, the top of at least one of the pattern elements opposite the first or second point,    ii) at the given moment a reference value is measured, which is representative of a distance measured between the first or second point and the bottom of one of the grooves opposite the first or second point, and    iii) the value measured in paragraph i) is compared with the reference value measured in paragraph ii), and from this is deduced a wear datum representing the height of the pattern element by differential measurement at the moment.
According to this embodiment of the invention, the method comprises for example, at any moment or at all times, the following steps:    from the first or second point an incident beam of acoustic or electromagnetic waves (such as ultrasonic waves or light waves, respectively) is emitted towards at least one of the grooves opposite, and towards at least one of the tread element tops adjacent to it,            on the one hand a first beam of waves originating from the incident beam and reflected by the tread element top, and on the other hand a second beam of waves originating from the incident beam and reflected by the groove bottom, are received in the form of signals,        from the shift between the signals, the delay time of the signal corresponding to the second beam relative to the signal corresponding to the first beam is deduced, and        from this, the distance between the tread element top and the groove bottom, representing the height of the tread pattern element(s) at the moment or at all times, is deduced.        
Also according to this aspect of the invention, the method advantageously uses acoustic waves such as ultrasonic waves as the waves.
Again according to this embodiment of the invention, the method comprises the following steps:                in a continuous way over at least one revolution of the tire during the rolling of the vehicle, on the one hand n measurements (n>1) are made which are representative of the distances between the first or second point and the respective tops of several of the tread pattern elements opposite the point, and on the other hand n reference measurements are made which are representative of the distances between the first or second point and the bottom of grooves adjacent to the pattern elements,        n comparisons are made between these measurements, from which are deduced, by differential measurements, n momentary wear data representing the height of the pattern elements, and        an average of these n momentary wear data is formed, from which is deduced a datum relating to the average wear of the tire over its entire circumference during rolling.        
According to an aspect of the first example embodiment of the invention, the method also comprises the step of displacing the first point fixed on the suspension system in a direction perpendicular to the median circumferential plane of the tire, to measure the wear of a plurality of tread pattern elements in the axial direction of the tire.
According to another characteristic of the first embodiment of the invention, the first point is attached to a suspension arm of the suspension system.
According to a further characteristic of the first embodiment of the invention, the first point is attached to an abutment against which rests a damper spring of the suspension arm.
According to a further example embodiment common to the first or second embodiments of the invention, the method comprises the step of measuring the values representative of distances by means of a wear gauge, such as a feeler, which is mounted on the first or second point, controlling the movement of the feeler so that it comes in contact with one of the tread element tops and/or with the groove bottom, and then measuring the displacement of the gauge and comparing that displacement with a reference displacement, to deduce therefrom a wear datum of the tread pattern element(s).
Note that wear gauges other than feelers could just as well be used, for example “combs” whose face intended to contact the tread has a profile substantially conforming to that of the latter.
Another purpose of the present invention is to propose suspension systems for implementing this wear measurement method according to the invention, each such suspension system being intended for fitting on an automobile vehicle comprising a chassis and wheels, each wheel being provided with a tire comprising a tread with pattern elements, the suspension system being designed to provide a flexible link between the chassis and the wheels.
To this end, according to a first example embodiment of the invention a suspension system is provided with a wear measurement unit comprising a wear gauge, such as a feeler, the gauge being mounted on a fixed abutment of the suspension arm against which rests a damper spring, in such manner that the gauge can make contact with the tread element top and/or the groove bottom of at least one of the tread pattern elements under the guidance of control means also mounted on the suspension system, the wear measurement unit also comprising means for measuring the displacement of the gauge and for comparing that displacement with a reference displacement and deducing therefrom a wear datum pertaining to the pattern element(s).
As the control means, any means can be used, for example, electric, pneumatic, electro-pneumatic or electromagnetic, in particular, such as jacks.
A suspension system according to a second example embodiment of the invention is provided with a wear measurement unit comprising a pick-up for acoustic or electromagnetic waves, such as ultrasonic or light waves respectively, the pick-up being mounted on a fixed abutment of the suspension arm against which rests a damper spring, and being designed to emit at a certain moment or at all times an incident beam of the waves towards at least one of the grooves opposite it and towards at least one of the tops adjacent thereto, the unit comprising, in addition:                means for receiving in the form of signals, respectively, on the one hand a first beam of waves reflected by the top and originating from then said incident beam, and on the other hand a second beam of waves reflected by the groove bottom and originating from then said incident beam,        means for deducing from the shift between the signals, the delay time of the signal corresponding to the second wave beam relative to the signal corresponding to the first wave beam, and        means for deducing therefrom the distance between the top and the groove bottom, which represents the height of the tread pattern element(s) at the moment or at any time.        
An automobile vehicle according to the invention comprises a chassis, wheels and a suspension system which provides a flexible link between the chassis and the wheels, a tire whose tread comprises pattern elements interconnected by grooves being fitted on each of the wheels.
According to an example embodiment of the invention, the vehicle is such that its suspension system has the characteristics defined earlier, and the vehicle also has a central unit which is designed to receive the wear data pertaining to at least one of the tires coming from the suspension system and is capable of informing the driver of the vehicle about the wear of the tire at any time.
According to another example embodiment of the invention, the vehicle is such that its chassis is provided with a wear measurement unit comprising a pick-up for acoustic or electromagnetic waves, such as ultrasonic or light waves respectively, the pick-up being mounted opposite the tire on a longitudinal member of the chassis or a wall of the chassis that corresponds to the wheel passage zone and being designed to emit, at a given moment or continuously, an incident beam of the waves towards at least one of the pattern grooves opposite it and at least one of the tread element tops adjacent thereto, the unit comprising, in addition:                means for receiving in the form of signals, respectively, on the one hand a first beam of waves reflected by the tread element top and originating from the incident beam, and on the other hand a second beam of waves reflected by the groove bottom and originating from the incident beam,        means for deducing from the shift between the signals, the delay time of the signal corresponding to the second wave beam relative to the signal corresponding to the first beam, and        means for deducing therefrom the distance between the tread element top and the groove bottom, which represents the height of the pattern element(s) at the moment or at any time,        
and the vehicle also has a central unit which is designed to receive the wear data pertaining to at least one of the tires coming from the measurement unit, and is capable of informing the driver of the vehicle about the wear of the tire at any time.
Note that when the wave pick-up is mounted on a longitudinal member of the chassis or on a wall of the chassis corresponding to the wheel passage zone (for example, on the vehicle's mudguard adjacent to the tire), the pick-up oscillates during the rolling of the vehicle practically in just one direction which corresponds to the movements imposed by the suspension system when absorbing the load. Thus, these pick-up oscillations have no effect on the aforesaid differential measurements made at a given moment between the bottom of a groove and the top of an adjacent pattern element.
Advantageously, the vehicle according to the invention is such that at least one of its tires has all or part of its tread pattern element tops and/or grooves in the form of concave surfaces such as paraboloids of revolution.
Note that this particular geometry for the tire pattern tops and/or grooves optimizes their reflection of the electromagnetic or acoustic waves emitted by the measurement unit according to the second embodiment mentioned above, and consequently, optimizes the precision of the results obtained.